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2. Geomagnetism and Paleomagnetism 2-1 2. GEOMAGNETISM AND PALEOMAGNETISM 1 https://physicalgeology.pressbooks.com/chapter/4-3-geological-renaissance-of-the-mid-20th-century/ 2 2-2 ELECTRIC q Q FIELD q -Q 3 MAGNETIC DIPOLE Although magnetic fields have a similar form to electric fields, they differ because there are no single magnetic "charges," known as magnetic poles. Hence the fundamental entity is the magnetic dipole arising from an electric current I circulating in a conducting loop, such as a wire, with area A . The field is described as resulting from a magnetic dipole characterized by a dipole moment m Magnetic dipoles can arise from electric currents - which are moving electric charges - on scales ranging from wire loops to the hot fluid moving in the core that generates the earth’s magnetic field. They also arise at the atomic level, where they are intrinsic properties of charged particles like protons and electrons. As a result, rocks can be magnetized, much like familiar bar magnets. Although the magnetism of a bar magnet arises from the electrons within it, it can be viewed as a magnetic dipole, with north and south magnetic poles at opposite ends. 4 2-3 MAGNETIC FIELD 5 We visualize the magnetic field of a dipole in terms of magnetic field lines pointing outward from the north pole of a bar magnet and in toward the south. The lines point in the direction another bar magnet, such as a compass needle, would point. At any point, the north pole of the compass needle would point along the DIPOLE field line, toward the south pole MAGNETIC of the bar magnet. The earth’s magnetic field, as discussed FIELD shortly, looks essentially like a dipole field. A tricky point is that the earth’s field is that of a dipole pointing south along the earth’s axis, so the magnetic pole near the geographic North pole is a south magnetic pole. We call this the North magnetic pole. because the north pole of a compass needle, which is a north magnetic pole, points toward it 6 2-4 DYNAMO GENERATES MAGNETIC FIELD Current produced by Lorentz force on charged particles moving with velocity v in magnetic field B F = qv x B 7 Davidson et al The solid inner core is GEODYNAMO roughly the size of the moon but at the temperature of the surface of the sun. Convection in the fluid outer core is thought to be driven by both thermal and compositional buoyancy sources at the inner core boundary that are produced as the Earth slowly cools iron in the iron-rich fluid alloy solidifies onto the inner core giving off latent heat and the light constituent of the alloy. These buoyancy forces cause fluid to rise and Coriolis forces, due to the Earth's rotation, cause the fluid flows to be helical. http://www.abc.net.au/science/basics/img/earth_spins.jpg 8 2-5 A snapshot of the 3D magnetic field structure simulated with the Glatzmaier-Roberts geodynamo model. Magnetic field lines are blue where the field is directed inward and yellow where directed outward. The rotation axis of the model Earth is vertical and through the center. A transition occurs at the core-mantle boundary from the intense, complicated field structure in the fluid core, where the field is generated, to the smooth, potential field structure outside the core. The field lines are drawn out to two Earth radii. https://websites.pmc.ucsc.edu/ ~glatz/geodynamo.html 9 About 36,000 years into the simulation the magnetic field underwent a reversal of its dipole moment, over a period of a little more than a thousand years. The intensity of the magnetic dipole moment decreased by about a factor of ten during the reversal and recovered immediately after, similar to what is seen in the Earth's paleomagnetic reversal record. Our solution shows how convection in the fluid outer core is continually trying to reverse the field but that the solid inner core inhibits magnetic reversals because the field in the inner core can only change on the much longer time scale of diffusion. Only once in many attempts is a reversal successful, which is probably the reason why the times between reversals of https://websites.pmc.ucsc.edu/ the Earth's field are long10 ~glatz/geodynamo.html and randomly distributed. 2-6 EARTH’S FIELD 11 EARTH’S FIELD GEOCENTRIC AXIAL DIPOLE (GAD) 12 2-7 INCLINATION AND LATITUDE 13 The real field isn’t a pure dipole 14 2-8 ACTUAL FIELD GEOMETRY Although the inclined dipole is a better description of the field, neither it nor any other dipole fully describes the field. The remaining part of the field, the non-dipole field, is about 10% of the total field. 15 The magnetic field changes with time, presumably because of changing fluid motions in the core. Changes on time scales less than 100,000 years are called secular variation. 16 2-9 Thermal Rocks Remnant Magnetization record earth’s field when they were magnetized Thermal Remnent Magnetism (TRM), results when molten volcanic rock cools. Basalt and other volcanic rocks contain iron and titanium bearing minerals like magnetite (Fe3O4), hematite (Fe2O3), and ilmenite (FeTiO3). These are ferromagnetic materials, whose atomic structure causes regions of parallel dipoles called magnetic domains. Normally, the domains are randomly oriented, so the material has no net magnetization. However, applying an external magnetic field aligns the domains and reorganizes them, giving a net magnetization. This ordering persists after the external field is removed. However, the ordering is destroyed if the material is heated above its Curie temperature, because thermal oscillations overcome the domain alignments. The process occurs in reverse as molten igneous rocks cool, first becoming solid at about 800 – 1100 C, and eventually cooling through the Curie temperature of the magnetic minerals, about 600 C. Below the Curie temperature minerals retain a magnetization that records the earth's field when and where they cooled, even when the earth's field changes and the rock is transported on a moving plate. 17 Detrital Remnant Magnetization This occurs when sediment is deposited in water still enough for magnetized grains to be aligned by the earth's field. The earth's field exerts a torque that rotates grains' magnetic moments toward the direction of the field. How this actually occurs during the complex process of sediment deposition - that involves a variety of grains with different sizes, only some of which are magnetic, subject to a number of processes including biological perturbations - remains an area of research. Paleomagnetic data from deep sea sediments provides important data about the evolution of the ocean basins. DRM typically 1-2 orders of magnitude weaker than TRM. 18 2-10 Chemical Remnant Magnetization 19 20 2-11 Seafloor tape recorder Cooling dikes at midocean ridges acquire earth's magnetic field Seafloor’s magnetic anomalies record spreading history Find ages and spreading rate from known magnetic reversal history Davidson 7.7 21 22 Uyeda 2-12 Seafloor magnetic anomalies give ages 23 Davidson 7.8 24 2-13 SPINNER MAGNETOMETER 25 SUPERCONDUCTING MAGNETOMETER 26 2-14 Paleomagnetic pole, or "apparent pole" Imagine a rock formed at the equator, which thus has a horizontal magnetic field. If it is moved to another place - for example 45°N - its magnetization will not be parallel to the field there. The rock's magnetization shows that it is now 45° away from where it was magnetized. The paleomagnetic pole, or "apparent pole", inferred from the rock is 45° degrees from the current North pole. This apparent polar wander (APW) results primarily from the rock, not the pole, having moved. Apparent pole positions from rocks formed at different times, but on the same plate, can be used to construct an APW path through time. 27 28 2-15 29 30 2-16 31 32 Tettegouche State Park, MN 2-17 Midcontinent Rift (MCR) Prominent on gravity & magnetic anomaly maps Long arms of buried dense & highly magnetized 1.1 Ga igneous rocks ~ 3000 km long ~ 2 x 106 km3 magma Outcrops near Lake Superior 33 MCR likely formed as part of the rifting of Amazonia from Laurentia, recorded by APWP cusp & became inactive once seafloor spreading was established APWP for Laurentia poles Stein et al., 2014 34 2-18 Laurentia’s apparent polar wander path (APWP) has abrupt cusp at ~1.15 Ga before major MCR igneous activity starts Cusps indicate change in direction associated with rifting 35 Stein et al., 2014 Schettino and Scotese, 2005 .
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